The present invention generally relates to pyrotechnic delay compositions that burn slowly to allow for a time lapse before ignition of a primary charge and, more particularly, to novel non-toxic pyrotechnic delay compositions in which no ingredients pose any environmental hazard.
A delay fuse is a known pyrotechnic device designed to give a delay before ignition of a primary charge, or between ignitions of successive charges in an explosive train. Pyrotechnic delay fuses are widely employed in fireworks exhibitions, mining, quarrying and other blasting operations in order to permit sequential initiation of the explosive charges in a pattern. They are also commonly used in artillery applications to afford a number of seconds for the operator to retire from the artillery before it functions, or to time the explosion of an artillery shell.
Existing delay detonator cartridges comprise a metallic shell closed at both ends and containing in sequence a percussion cap, pyrotechnic delay composition, and igniter. The delay composition imposes an ignition delay between the percussion cap and igniter.
FIG. 1 is an illustration of a typical delay cartridge 13. The cartridge 13 (sometimes referred to as a “shell” or “cartridge shell”) is screwed into artillery and is sealed by an O-ring 20. The cartridge 13 includes a percussion cap including a primer 12, which is loaded into cartridge 13, and held by a primer holder 10. The percussion primer 12 is in communication through a calibrated orifice disk 14 with an AlA igniter mix 22. A pyrotechnic delay composition 24 is contained after the igniter mix 22, and this composition 24 is held next to an 80-20 (or equivalent) igniter mix 26. Igniter mix 26 is in communication through another orifice disk 14 to output charge(s) 16, which is sealed in its end of the cartridge 13 by a closure disk 18. Thus, when the percussion cap is detonated, primer 12 is ignited and this ignition ignites the AlA ignition mix 22, which in turn ignites delay composition 24. After the pyrotechnic delay composition 24 has burned through, the flame reaches the 80-20 igniter mix 26, which combusts the output charge(s) 16, whereupon the payload explodes.
A large number of burning pyrotechnic delay compositions are known in the art, and generally include mixtures of fuels and oxidizers. There are certain requirements for these compositions. They must burn without creating large amounts of gaseous by-products which would interfere with the functioning of the delay detonator. Moreover, pyrotechnic delay compositions should be safe to handle, from both an explosive and health perspective, and they must be resistant to moisture and degradation over periods of time. They are also subject to volume constraints as they must operate in a wide range of delay detonators within the confines of space available inside existing detonator shells.
A large number of delay compositions consisting of mixtures of fuel and oxidizers are known, e.g. Manganese Delay (MIL-M-21383: Mn—PbCra4—BaCrO4), Tungsten Delay (MIL-T-23132: W—BaCrO4—KClO4—SiO2), T-10 (B—BaCrO4,), etc. See, e.g., M. E. Brown, S. J. Tylor, and M. J. Tribelhorn, Fuel-Oxidant Particle Contact in Binary Pyrotechnic Reactions, Propellants, Explosives, Pyrotechnics 23, 320-327 (1998). Unfortunately, these existing ignition delay mixtures are not environmentally friendly due to the toxicity of individual components. For example, Manganese Delay (MIL-M-21383) or Tungsten Delay (MIL-T-23132) and other similar pyrotechnic delay compositions contain carcinogenic hexavalent chromates. Silicon and barium sulphate delay compositions include a proportion of red lead oxide, also carcinogenic. There is a significant desire in the explosives industry to eliminate all use of lead or other toxins and carcinogenics as compounds in delay compositions.
Recently it was found that Si—Al—Fe3O4 could be considered as a potential replacement for commercial formulations. This mixture appears to be very safe when tested by impact or friction and it is rather insensitive to electrostatic discharge. Ignition temperatures are close to 1000° C. The advantages of Si—Al—Fe3O4 are its insolubility in water and resistance to moisture and that it is environmentally benign. Thus, it would be greatly advantageous to provide a non-toxic pyrotechnic delay composition based principally on Si—Al—Fe3O4 that burns substantially gas-free, is safe to handle, is resistant to moisture and degradation over time, can be incorporated within the confines of existing detonator shells, and that poses no environmental hazard.